1. Field of the Invention
This invention relates to a process for regenerating an antimony containing oxide catalyst, and more particularly to a process for regenerating an antimony containing oxide catalyst the activity of which has been deteriorated as a result of its use in the oxidation, ammoxidation or oxidative dehydrogenation of hydrocarbons.
2. Description of the Prior Art
It is well known that metal oxide compositions comprising as essential components (i) antimony and (ii) at least one metal element selected from the group consisting of iron, cobalt, nickel, manganese, uranium, cerium, tin and copper is useful as a catalyst for the oxidation, ammoxidation or oxidative dehydrogenation of hydrocarbons, for example, it can be advantageously used in the production of unsaturated aldehydes through the oxidation of olefins, the production of unsaturated nitriles through the ammoxidation of olefins, and in the production of diolefins through the oxidative dehydrogenation of olefins. As catalysts for the production of acrylonitrile by the ammoxidation of propylene, Japanese Patent Publication No. 19111/64 discloses a catalyst comprising an oxide of antimony and iron, cobalt or nickel, U.S. Pat. No. 3,152,170 discloses a catalyst comprising an oxide of antimony and tin, U.S. Pat. No. 3,308,151 discloses a catalyst comprising an oxide of antimony and uranium, and U.S. Pat. Nos. 3,200,081 and 3,340,291 disclose a catalyst comprising an oxide of antimony and manganese or copper. Some later improvements on these catalysts are disclosed in U.S. Pat. No. 3,668,147, Japanese Patent Publication No. 40985/72 and Japanese Patent Publication No. 19764/72 wherein tellurium and at least one element selected from the group consisting of molybdenum, vanadium and tungsten is incorporated in catalysts comprising oxides of antimony and iron, antimony and tin, and antimony and uranium, respectively. Japanese Patent Publication No. 40957/72 describes a catalyst comprising an oxide of antimony and at least one element selected from the group consisting of cerium, titanium, manganese, cobalt, nickel and copper.
In spite of their good catalytic performance, none of the above catalysts are fully satisfactory upon prolonged use and their service life is not always sufficiently long. Even the improved catalysts are subject to a gradual decrease in their activity with extended use and improper reaction conditions often accelerate a reduction in the catalytic activity. It is economically unfeasible to continue using a catalyst which has had its activity reduced below a certain level. In particular, when the catalyst is used industrially on the large scale as in the preparation of acrylonitrile, the influence caused by the deterioration is large and when the deteriorated catalyst is not replaced with a fresh one in the appropriate time, the economical loss is caused remarkably. However, since catalysts of the above specified type are expensive, it is a substantial expenditure for the manufacturer to replace the deteriorated catalyst with a fresh one. It would therefore be economically advantageous if a practical method for regenerating the catalyst were available.
As will be understood from the above explanation, one criterion for determining whether a catalyst is deteriorated or whether a detriorated catalyst has been regenerated by a regenerative method is on an economic level, in contrast to a technical level, which takes into account the activity and selectivity of the catalyst. Based on experience a catalyst is considered "deteriorated" if the yield of the end product is reduced by more than 2 to 3% of the yield obtained using the fresh catalyst, and a catalyst is considered "regenerated" if such yield is restored to the original level or higher.
It is difficult to enumerate the causes of deterioration of a catalyst which occurs during its use. In most cases, many factors combine to cause such deterioration, what is more, locating a particular contributing factor does not directly lead to the development of an effective method of regenerating the catalyst. Therefore, many attempts at providing effective regeneration of the catalyst have turned unsuccessful.
A method of regenerating an antimony-uranium oxide catalyst is described in U.S. patent application Ser. Nos. 83,187 and 103,005 (corresponding to Japanese Patent Application (OPI) No. 8615/72) (The term "OPI" as used herein refers to a "published unexamined Japanese patent application") and British Pat. No. 1,365,096. That method is characterized by heating an antimony-uranium oxide catalyst complex in a fluidized state, in a non-reducing gas at a temperature of from 800.degree. to 1,800.degree. F. and for a time such that the surface area of the catalyst does not fall below a minimum critical level of 5 m.sup.2 /g. The basic concept behind the method is to heat the catalyst before its performance, which is determined by the surface area of the catalyst, drops to a minimum critical level. In addition, the method is applicable over a wide range of temperatures. Therefore, this method may be considered a satisfactory practical method for regenerating the catalyst. However, antimony containing oxide catalysts wherein antimony is combined with elements other than uranium cannot be regenerated by such a simple procedure. For example, it is described in U.S. patent application Ser. No. 954,675 (corresponding to Japanese Patent Application (OPI) No. 62193/79) that iron-antimony containing oxide catalysts which have a specific composition can only be regenerated under relatively limited conditions and only when the deteriorated catalyst has a specified nature. Accordingly, the method described in Japanese Patent Application (OPI) No. 8615/72 is only suitable for regenerating of an antimony-uranium oxide catalyst.
U.S. Pat. No. 4,049,575 discloses a novel processes for the production and the improvement of catalysts including the catalyst which may be regenerated in accordance with the process of this invention. In accordance with the process disclosed in U.S. Pat. No. 4,049,575 a catalyst composition is prepared by impregnating or spraying onto a mixed metal oxide composition consisting of antimony and a specific metal with a solution containing other active components. The process can be advantageously used in regenerating a deteriorated catalyst as demonstrated by some of the working examples in the patent, but the method is rather complex and costly because it involves preparing a solution of the catalytic component with which the catalyst is impregnated, impregnating the catalyst with a predetermined amount of the solution, drying, and calcining the impregnated catalyst. In particular, the method requires that the impregnating solution contain at least two catalytically active components, but it is not easy to prepare one stable impregnating solution which does not produce a precipitate, for instance. As a result, it is sometimes necessary to use rather expensive reagents as starting materials of active components. The method also introduces new catalytic components to the catalyst, thus yielding a regenerated catalyst having a different composition and different physical properties than the original catalyst or having a different reaction rate and different optimum reaction conditions. Therefore, it is often difficult to use the catalyst regenerated in this way in combination with the fresh (unregenerated) catalyst without some disadvantage.
Further, Japanese Patent Application (OPI) No. 81191/79 (corresponding to U.S. patent application Ser. No. 959,810, filed Nov. 18, 1978) provides a process for regenerating an antimony containing oxide catalyst, which comprises impregnating or spraying onto the deteriorated antimony containing oxide catalyst an aqueous solution of nitric acid and/or a nitrate and then drying the impregnated metal oxide catalyst followed by calcining the impregnated catalyst at a temperature ranging from 400.degree. to 1,000.degree. C. The method disclosed in Japanese Patent Application (OPI) No. 81191/79 can be conducted with a wide range of catalysts to be regenerated and is a comparatively simple process in the point of the regenerating operations and conditions. However, because nitric acid and/or the nitrate is very corrosive, the materials used for the regenerating apparatus are extremely restricted and as the large amounts of a nitric acid and a nitrogen oxide are present in the waste gas, the waste gas cannot be vented to the outside, the process has the disadvantage that the apparatus for treating the waste gas must be equipped with pollution control devices to avoid a pollution problem. Therefore, while the method is comparatively simple to conduct, the method has economical problems associated with industrial use. On the other hand, in order to overcome the above problems with respect to the regeneration of the catalyst, this invention was achieved.